1. Field of the Invention
The object of this invention is to provide a method for measuring the concentration of solid or liquid particulate matter suspended in a gaseous carrier medium; a particular application being the measurement of concentration and flow rate of pulverised solid fuel pneumatically transported in a duct to power station boiler burners.
2. Description of the Prior Art
There already exist methods for measuring the concentration of particulate matter in a gaseous carrier medium: these exploit the effect of particulate matter concentration on the parameters of radio frequency (RF) electromagnetic radiation transmitted along the duct transporting the particulate matter. Also, the established physical principle states that at a precisely specified frequency, known as the cut-off frequency, a step change in the electromagnetic radiation parameters can be detected—the step change being a transition from attenuation to propagation mode. The value of the cut-off frequency depends upon the physical properties of the duct and on the waveguide geometry.
One specific method of measurement utilising the principle described above is presented in a Polish patent application no. P-337795 (WO9901752). This particular method is based on an RF electromagnetic wave being transmitted along a duct (which acts as a waveguide). Subsequently, at a point in the duct axially remote from the transmission point, the attenuation of the signal is continuously measured at a frequency below the cut-off frequency. The change of the cut-off frequency value is derived by making a mathematical comparison between a range of attenuation curves defined for the actual measurement conditions and a range of attenuation curves obtained under calibration conditions for the flow of particulate matter of known concentration. Usually, the calibration process is carried out using clean air. Subsequently, the dielectric permittivity ∈1 of mixed carrier gas and suspended particulate matter is derived from the following formula:∈1=1/μ1(1−Δfg/fgo)2,which is derived from the conversion of the following equation:Δfg=fg0−fg1=fgo(1−1/√μ1∈1),wherein:    Δfg—the cut-off frequency shift between calibration conditions fgo and actual measurement conditions fg1     μ1—magnetic permeability of mixed carrier gas and suspended particulate matter.
Thus from the derived dielectric permittivity ∈1, a corresponding value for the concentration of the particulate matter can be calculated.
In case of the aforedescribed method, the accuracy with which the concentration of particulate matter within the duct or waveguide depends on the accurate estimation of cut-off frequency values. Attenuation measurement is based on measuring and comparing the amplitude of the RF electromagnetic wave at a point where it is generated and at a second point axially remote from the first position. The achievable accuracy of these measurements proves insufficient to properly determine the flow rate of pneumatically transported particulate matter. Furthermore, it must be pointed out that the “synthesized” relationship between the measured attenuation and the shift in cut-off frequency, as it is represented by the derived mathematical model curves, is not identical to the actual relationship observed by comparing the cut-off frequency shift caused by the change of concentration of particulate matter in actual flow conditions with the flow conditions measured when the concentration of particulate matter is known. The slope of these substantially linear sections is a function of dielectric permittivity and conductivity of the two-phase flow: if the linear sections are out-of-parallel, this may yield strongly distorted measurement results.